1. Field of the Invention
This invention relates to the use of dihydroxyfatty acids and/or dihydroxypolyfatty acids as isocyanate-reactive compounds bearing an ionizable group for the production of carboxyfunctional polyurethanes. The invention also relates to polyurethanes produced using hydroxyfatty acids and to a process for their production.
2. Discussion of Related Art
The dihydroxyfatty acids suitable for use in accordance with the invention are known from DE-OS 33 18 596. It is known from DE-OS 35 07 505 that polyether polyesters based on the dihydroxyfatty acids mentioned can be used as a polyol component in the production of polyurethanes. In their case, however, the carboxylic acid function is esterified so that neither the polyol nor the polyurethane produced therefrom contains ionizable carboxy functions. Accordingly, the dihydroxyfatty acids are not reacted with the isocyanates as a free reactive component, but instead are used for the production of an intermediate product, namely the polyether polyester.
Dihydroxycarboxylic acids are generally known to the expert as suitable hydrophilic structural elements for polyurethanes from earlier applications filed by applicants, for example from DE-OS 38 27 378. When specified in detail, these hydroxycarboxylic acids have a relatively low carbon number, i.e. are not derived from natural fatty acids. In addition, it is pointed out that, of the dihydroxycarboxylic acids mentioned in the literature, those in which the carboxy group is sterically hindered by at least one part of the molecule in the alpha-position thereto are preferred or are the only dihydroxycarboxylic acids described in Examples. DE 28 04 609 mentions suitable hydroxycarboxylic acids and states that similar acids containing sterically hindered carboxy groups and free unhindered hydroxyl groups are also suitable. Sterically hydroxycarboxylic acids containing up to 12 carbon atoms in the alkyl radical are also mentioned in GB 2,104,085.
A dihydroxycarboxylic acid preferred in almost every relevant document is dimethylol propionic acid (DMPA) which is sterically hindered to a particularly high degree. The preference for DMPA would appear to be attributable to the fact that the steric hindrance of the carboxy group--caused by the methylol groups in the alpha-position--greatly reduces its tendency towards secondary reactions, i.e. crosslinking, in relation to a corresponding compound containing a sterically unhindered acid group. However, this does give rise to certain process-related disadvantages in the production of the polyurethanes. The main reason for these disadvantages is that DMPA is generally solid at the reaction temperature and, accordingly, has to be size-reduced beforehand and that the reaction can only take place at the liquid/solid interface, i.e. in a two-phase system. This generally leads to relatively long reaction times, to residues of unreacted DMPA and to the often undesirable fact that a large part of the DMPA introduced only reacts after the hydroxyfunctional compounds have reacted off in the liquid phase, so that the carboxy groups are not uniformly distributed throughout the polyurethane skeleton. In addition, the hard segment character of DMPA--due to the relatively small size of the molecule coupled with high functionality--causes a relatively high viscosity of the polyurethane polymers or prepolymers and can also have an adverse effect on their performance properties, such as their resistance to water.